A liquid crystal panel (hereinafter referred to .pi. cell) having liquid crystals filled between one pair of facing substrates wherein crystals orient in such a quasi-parallel way that a pre-tilt direction of both substrate molecules are planar symmetrical against the center plane between substrates has certain characteristics, namely a short ON-OFF time (hereinafter referred to a response time) and an eminently low visual dependency (Philip J. Bos et al, SID 83 DIGEST P. 30-31, 1983).
Furthermore, a flat liquid crystal image display panel can be obtained by switching selectively an active switching element formed on either transparent substrate in .pi. cell in matrix form (Toku-Kai-Sho 61-116329).
Also, in .pi. cell, an OFF state (a bend orientation) is quasi-stable and may shift to a more stable quasi-homogeneous state (a splay orientation) within a relaxation time of several seconds to several dozen seconds. In order to carry out switching an ON state (a vertical orientation) and an OFF state stably, it is necessary to put a certain bias voltage before a bend orientation of an OFF state shifts to a splay orientation. Concretely, it may be a periodical ON voltage or an irregular pulse which does not obstruct displaying (Toku-Kai-Sho 61-128227).
Furthermore, those with a more improved visual dependency obtained by placing .pi. cells and a birefringent film between crossnicols are known as OCB cells (Y. Yamaguchi et al., SID 93 DIGEST P. 227, 1993).
In OCB cells, there are used cells with twist angles between upper and lower substrates of 0.degree. (the above-mentioned.pi. cells), biaxial phase different plates and nematic liquid crystal compositions without addition of any chiral agent, which are very different from cells of conventional TN (twist nematic) and STN (supertwist nematic) modes. A characteristic of the OCB cells on driving is to drive liquid crystal molecules in cells in a bend oriented state. That is, it is realized to obtain a very wide visual angle and a very high contrast compared to cells of conventional TN (twist nematic) and STN (supertwist nematic) modes by combining bend oriented liquid crystals with biaxial phase different plates. Furthermore, it is also realized to obtain a very fast response time such as 1/20 to 1/500 compared to cells of conventional TN (twist nematic) and STN (supertwist nematic) modes by driving liquid crystals in a bend oriented state.
However, any combination of OCB cell and an active switching element is not known from the above-mentioned prior arts.
Particularly in the case of using OCB cells combined with active switching elements, it is not known that characteristic combinations of materials, for example, liquid crystal materials, oriented membranes etc., influence display qualities of the cells.
For example, liquid crystals used in the above-mentioned prior arts are ZLI-1132 made by Merck Co. consisting of cyanophenylcyclohexane derivatives and E-44 made by BDH Co. consisting of cyanobiphenyl derivatives, etc. Furthermore, there is no description about an oriented membrane.
In the case that a flat liquid crystal image display panel is prepared by switching selectively an active switching element in .pi. cells, particularly formed on either transparent substrate of .pi. cells in matrix form, it is known that deterioration of image signal results in deterioration of image display characteristics, especially the contrast.
Deterioration of image signals depends on direct current residual electric charges or voltage holding ratios of constituting materials, especially liquid crystal materials and oriented membrane materials, arranged within insides of cells. For example, the above-mentioned ZLI-1132 or E-44 uses compounds having CN groups as terminal groups much, so that not only direct current residual electric charges become more, but also voltage holding ratios become very less.
As a result, these liquid crystal materials cause a decrease in the contrast of display in an active matrix driving method, which can not be put to practical use.
However, combinations of various parameters attained are still unsuitable and insufficient especially as to an electric optical system according to the present invention having a high contrast. It attributes to the fact that various requirements are influenced oppositely due to physical properties.
Thus, in order to have a high contrast and simultaneously possess a wide active temperature range, a good high-speed response, a wide visual range, a low temperature dependency of a threshold voltage value and a low frequency dependency adapted to the above-mentioned requirements, there are many problems to be solved.